I stole the idea from Bowery Lane Bicycles. Every time they posted new photos of their bikes, I found I was only looking at the cool boxes they have on the back. Here's what I'm talking about: http://bowerylanebicycles.com/broncks_black.html
It took a bit of looking to find the right box. After a beautiful meal at Enotria, their wine manager kindly gave me a box from a case of Far Niente winery. It proved too beefy to go on a rear rack, but it's a gorgeous box that will find some other cool use.
Eventually, I found the right size box at a local wine merchant and the owner sold me four of them for a very fair price. Here's the box in question:
And here is the box being examined by my assistant, Scamp.
Here's Takumashi wearing his new storage gear:
For this project, I drilled two holes in the bottom of the box and in a thin strip of wood that went under the box and rack. I ran bolts through the box, rack and strip, and fastened lock nuts under the strip. This is very sturdy... You can lift the bike by the box and not disturb its position on the rack.
Here's Roebuck, wondering why he's been posed next to a swimming pool.
Total cost for each bike was about $3.25. The boxes do their job well. I put a golf towel in the bottom to keep the lock and other stuff from bouncing around too much. One slight drawback is that the box prevents use of the saddle loops and a saddle bag with repair kit. More on overcoming that another time.